About four million people who are now in the country illegally didn't climb walls or trek through the desert to get here. They came in legally, with tourist or student visas. When their visas expired, they chose to stay anyway. Congress will try to address this huge problem as it grapples with immigration legislation. NPR's Ted Robbins reports on some of the challenges of an exit visa system.

TED ROBBINS, BYLINE: Welcome to the U.S., jetlagged traveler.

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UNIDENTIFIED WOMAN: What's the purpose of your trip today?

UNIDENTIFIED MAN: I'm coming for a business conference.

UNIDENTIFIED WOMAN: And how long will you be in the United States?

ROBBINS: As a visitor, this CBP, or Customs and Border Protection, video points out you are now required to have your fingerprint and photograph taken.

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UNIDENTIFIED WOMAN: Can you take your glasses off for a photo, please?

ROBBINS: This program, up until recently called US-VISIT, is the largest biometric database in the world. Biometrics are what you call digital fingerprints and photographs in the world of security. Biometrics are more secure than paper because they're harder to tamper with or fake. They can be used to track criminals and potential terrorists, and biometrics can be used to keep track of visitors who might overstay their visas and become undocumented immigrants. But there's a flaw. Right now, US-VISIT only registers people when they arrive, not when they depart.

MARK KRIKORIAN: The only way you know which people have overstayed their visa is by knowing who has left and honored the terms of their visa.

ROBBINS: That's Mark Krikorian. He heads the Center for Immigration Studies, a Washington think tank which wants stronger immigration enforcement. He says an exit visa check would give the government a better idea of who the millions of people are who overstay their visas.

KRIKORIAN: You wouldn't necessarily be able to chase everyone down within 48 hours, but you'd at least know who had overstayed and you'd be able to take steps to deal with it.

ROBBINS: The Senate's current immigration bill mandates an exit visa check, but that's nothing new. It's mandated it six times in the last 17 years. With the exception of one unsuccessful pilot study at two airports, the exit mandate hasn't been fulfilled. Even the current Senate bill only requires an exit visa system for airports and seaports, not land ports, where three-quarters of the nation's visitors enter each year. Ports like this one at Nogales, Arizona. CBP supervisor Juan Osorio says that's because the situation here is a lot different than at an airport.

JUAN OSORIO: At an airport, you're in a controlled environment. You have a previous information list of who's coming on those flights.

ROBBINS: Here, vehicles are backed up into Mexico, sometimes for hours. As they approach U.S. Customs officers in lanes, a camera snaps a photo of each car's license plate. There's maybe 30 seconds lead time for the officer to figure out who's in front of him, and the license plate reader doesn't register everyone who's in the car.

OSORIO: So the officer has to enter each person's information into the computer. That's going to take some time.

ROBBINS: Jay Ahern is a former CBP commissioner who is now a private security consultant. He says putting in an exit visa checking system at airports and seaports will be tough. Putting it in land ports would require a huge effort similar to the buildup of fencing, technology and border patrol agents now in between the ports.

JAY AHERN: It's a staffing problem. It is a facility and logistics problem. It is a traffic flow problem.

ROBBINS: You'd need to build new lanes and new booths, and you'd need to staff them. Even with an exit system, you'd only know who is missing, not where they are, not unless you want DHS officers knocking on every door in the country.

AHERN: I think that that becomes a real serious consideration that people need to thoughtfully consider all the way through.

ROBBINS: That's not a politically palatable option in a free society, which may be one reason border militarization keeps getting the big bucks, even if it only addresses a little more than half the illegal immigration problem. Ted Robbins, NPR News, Tucson.

BLOCK: Tomorrow, we'll look at E-Verify, that's a program designed to prevent undocumented immigrants from being hired.

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